tests: remove dependencies on xglgpu.h

Replace XglGpu by xgl_testing::PhysicalGpu.  Replace MAX_GPUS by
XGL_MAX_PHYSICAL_GPUS.
diff --git a/tests/xglrenderframework.cpp b/tests/xglrenderframework.cpp
index cfe8a1c..8d0a303 100644
--- a/tests/xglrenderframework.cpp
+++ b/tests/xglrenderframework.cpp
@@ -56,7 +56,7 @@
     XGL_RESULT err;
 
     err = xglInitAndEnumerateGpus(&app_info, NULL,
-                                  MAX_GPUS, &this->gpu_count, objs);
+                                  XGL_MAX_PHYSICAL_GPUS, &this->gpu_count, objs);
     ASSERT_XGL_SUCCESS(err);
     ASSERT_GE(this->gpu_count, 1) << "No GPU available";